musl: meson: check existence of renameat2()

musl-1.2.5 does not provide renameat2(). Note, it is added by
https://github.com/kraj/musl/commit/05ce67fea99ca09cd4b6625cff7aec9cc222dd5a,
hence hopefully it will be provided by musl-1.2.6 or newer.

src/include/musl/stdio.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include_next <stdio.h>
+
+#if !HAVE_RENAMEAT2
+#  define RENAME_NOREPLACE (1 << 0)
+#  define RENAME_EXCHANGE  (1 << 1)
+#  define RENAME_WHITEOUT  (1 << 2)
+
+int missing_renameat2(int __oldfd, const char *__old, int __newfd, const char *__new, unsigned __flags);
+#  define renameat2 missing_renameat2
+#endif